If it runs decently forward and skips in backpedal, then its simply a misalignment. The chainline is what is involved in backpedaling. The difference in side to side alignment with the crank vs the cassette. The only thing in the equation worth even mentioning is the chain condition, but seriously there is little to be concerned with. Backpedaling is one thing they trained us out of as mountain bikers because it opens oneself up to problems such as that.